Many fields demand low-light imaging and video. Security cameras must often record minimally illuminated scenes; most marine environments receive little sunlight; the human body does not admit much of the light required for medical endoscopy.
A number of modern image sensor technologies have been developed to satisfy these demands. These modern low-light image sensors are, at their base, a digital camera. A digital camera replaces photosensitive film with an array of photoelectric sensors. The camera lens focuses an image onto the sensor array; each sensor produces an electrical signal proportional to the intensity of the light falling upon it; and the digital camera's processor converts the signals from the array of sensors into an image. To record the color of the light falling on a sensor, digital cameras typically include a color filter array (CFA) disposed over the sensor array. Each filter of the CFA transmits specific wavelengths of light. These digital cameras take color pictures by measuring the intensity of incoming light passing through the CFA. One example of a typical CFA is the RGB Bayer pattern, described in U.S. Pat. No. 3,971,065 and composed of red, green, and blue (RGB) filters in a checkerboard pattern. Red-filtered sensors measure the intensity of the red light that passes through the red filters, green-filtered sensors measure the intensity of the green light that passes through the green filters, and blue-filtered sensors measure the intensity of the blue light that passes through the blue filters. A digital camera processor typically interpolates the red, green, and blue intensity data to assign an RGB value to each sensor. Currently, several approaches within this digital camera paradigm have been used to produce digital cameras in general, and low-light cameras in specific. These methods include charge-coupled device (CCD) cameras and variations on active-pixel sensor cameras produced using the complementary metal oxide semiconductor process (CMOS cameras).
CCD cameras typically include a two-dimensional array of photosensitive elements or photoelectric sensors. Each element is coupled to a capacitor that can store light intensity information as a quantity of charge. One example of an image-sensing CCD is depicted in FIG. 3. The array of capacitors transfers the stored charge to a charge amplifier, which converts the charge into a readily measured voltage. The capacitors transfer charge down and across the CCD array until the entire CCD has been read. Most CCD cameras include an RGB Bayer CFA overlaid on the sensor array to discern colors.
Although CCD technology offers high light sensitivity and low noise, it suffers from several disadvantages. The charge generated in a CCD pixel array is transferred as an analog signal through only a few output nodes, requiring a higher data transfer rate and at least an analog-to-digital converter (ADC) outside the sensor. CCDs thus take up more space than comparable non-CCD sensors. Moreover, CCDs can consume up to 100 times as much power as comparable non-CCD sensors, generating heat that can degrade image quality. Additionally, light may be falsely recorded during the reading process: if light falls on a pixel while charge is being transferred between pixels, the light will increase the charge associated with a different pixel, erroneously creating a “smear” of light in the recorded image.
In contrast with CCDs, every pixel of a CMOS camera sensor includes its own amplifier. These active-pixel sensors can be made using the common complementary metal-oxide semiconductor (CMOS) process, and are thus commonly referred to as CMOS sensors. Like CCD cameras, CMOS cameras typically include an RGB Bayer CFA to discern color. The CMOS process allows more features to be added to the CMOS sensor. For example, image processing can be performed on the CMOS sensor chip, and adding an amplifier and an analog-to-digital converter to each pixel eliminates the data bottlenecks of CCD sensors. CMOS sensors thus take higher-resolution images more rapidly and efficiently than CCD cameras while eliminating smear. However, the additional circuitry surrounding each pixel in a CMOS sensor reduces the light sensitivity of CMOS sensors relative to CCDs, a particular disadvantage in low-light applications. To improve the low-light performance of CMOS sensors, researchers have developed several techniques, including frontside illumination (FSI) optimization, backside illumination (BSI), and direct CMOS systems.
A standard CMOS pixel places circuitry above the photosensitive layer. This circuitry scatters some of the light entering the CMOS pixel, reducing the effective sensitivity of the sensor. The FSI optimization technique uses a number of methods to reduce light scattering by the circuitry, including adding a waveguide to the pixel and applying antireflective coatings to the pixel array.
The BSI technique places the photosensitive layer directly behind the color filters and the circuitry behind the photosensitive layer. BSI may suffer from cross-talk, where data from one pixel creates noise by bleeding into nearby pixels. Both BSI and optimized FSI require relatively new manufacturing techniques. Moreover, despite the substantial changes in structure, both FSI and BSI techniques can claim no more than modest improvements in low-light conditions.
Direct CMOS cameras attempt to improve the low-light performance of CMOS sensors by replacing a single pixel covered by a red, green, or blue filter with a stack of three photodiodes embedded in silicon. Red, green, and blue light penetrate silicon to different depths, so different layers absorb primarily red, green, or blue light. A direct CMOS system thus acquires full-color information without a CFA, and thereby offers greater low-light sensitivity and higher resolution than a traditional RGB system. But each of the photodiodes is made of silicon: each photodiode absorbs overlapping portions of the spectrum. This overlap creates issues of color accuracy, particularly in low-light conditions. Moreover, as a direct CMOS system must acquire data for three different photodetectors per pixel, it is appreciably more difficult to manufacture than a standard CMOS system.
Accordingly, there is still a need for an improved low-light image sensor.